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Dimitrij Klingbeil Dimitrij Klingbeil is offline
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Default Reducing HV output voltage from Flyback/LOPT as used in arcademonitors

On 05.06.2018 00:30, John Robertson wrote:
I'm looking for a safe and effective way to reduce the voltage
output of a replacement flyback that is not original for the
circuit.

...

The danger - However what they are not aware of is the HV for a 19"
picture tube is 22.5 to 25KV and for a 13" colour tube it is
supposed to be 18.5KV to 22.5KV. Thus the 13" tubes are running at up
to 25KV which is 10% over their original maximum rating and my
concern is an increase of soft X-Rays. My HV probes show somewhere
around 25 to 30KV for the 19" flyback when used in a 13" chassis.


Hi

According to the numbers you provided, the EHT needs to be reduced by
about 7 kV (while keeping other voltages derived from the transformer at
their usual levels).

The 7 kV difference is significant, but not insurmountable for a series
dropper approach. The current draw of a CRT anode is small, the power
dissipation should still be manageable: 7 W at 1 mA (if the CRT ever
takes that much current).

There are TVS diodes with voltages up to 600 V (nominal value at 1 mA
test current) available (Littelfuse P6KE600A). A series string of 12
such TVS diodes will have around 7 kV of voltage drop across them.

The continuous power handling capability should be enough (each TVS of
this size can dissipate over 1 W, with heatsinking even multiple W).

The TVS diodes are reasonably cheap (some 40 cents a piece at Mouser for
QTY 10), so the main difficulty would probably be the insulation
(potting) of the series string. Probably best to put the whole thing
into a long insulating tube and fill the tube with epoxy.

Regards
Dimitrij